Twins and their non-twin siblings were initially recruited as part of ongoing studies of melanoma risk factors and cognition (McGregor et al., 1999; Wright et al., 2001). For the current study, processing speed and genotypic data, following quality control described below, were available for a maximum of 1659 individuals (from 730 families), of whom 266 were monozygotic twin pairs and for whom phenotypic data were averaged. When data were collected, participants ranged in age from 15.4 to 28.7 (mean = 16.4 years, SD = .8) with the sample being Caucasian, predominantly Anglo-celtic (i.e., ethnic outliers were excluded (Medland et al., 2009)). Fifty-three percent of the sample was female. Written informed consent was obtained from each participant and their parent/guardian (if younger than 18 years) prior to testing.
